This invention relates to a curable polymer emulsion composition containing a blend of an epoxy polymer and a polymer of vinyl acetate. In another aspect it relates to a stable epoxy-vinyl acetate polymeric emulsion which can be cured and cast at room temperature without the use of additional curative. In still another aspect it relates to a method of formulating an emulsion of epoxy resin and a vinyl acetate polymer with an amine curative to form a composition that is stable over long periods of time but can be cast and cured at room temperature. It also relates to the cast solid product obtained from such compositions.
Recent advances in polymer chemistry include the combination of thermosetting and thermoplastic polymers to take advantage of the properties of each type of polymer. For example, epoxy thermosetting polymers are often diluted with solvent in order to decrease viscosity for better coating or adhesive processing. For environmental and health reasons water-borne thermoplastic polymers are more acceptable, but these materials often have less water resistance and durability than their thermosetting counterparts. Combining epoxy resins with emulsion polymers such as the vinyl acetate polymers has, therefore, offered a good combination of properties including improved flexibility for the epoxy resin. The epoxy polymer, of course, must be cured with a suitable hardener, generally an amine, to reach its cross-linked state. Therein lies the problem with such polymer combinations because while the amine cross-linker can aid in water dispersion of the epoxy component, it also reduces the storage stability of the polymer mixture by causing premature coagulation. Solving this problem has occupied considerable attention in the polymer industry.
One approach to the problem has been to use what is known as a "two-pack" system. As described in U.S. Pat. No. 4,108,819, Oyamada et al. (1978) vinyl acetate and ethylene are copolymerized in the presence of an epoxide which is dissolved in the vinyl acetate before adding the ethylene under polymerization conditions. Otherwise, it is stated, the vinyl acetate-ethylene copolymer (VAE) and the epoxy resin are present heterogeneously leading to inferior products. The curing agent, such as xylylene diamine, is kept separate from the epoxyNAE combination until application. This is workable but is cumbersome since it requires special mixing at the point of use.
A procedure designed to overcome the "two pack" difficulty and develop a "one-pot" system is described by U.S. Pat. No. 5,177,122, Shih (1993). This rather complex procedure produces "core-shell" latices of epoxy resin and polymer of an ethylenically unsaturated monomer, such as vinyl acetate. As in the '819 patent, cited above, the vinyl acetate is polymerized in the presence of an epoxy resin to form the core of the composition. Then a hydroxyl or carboxyl containing monomer is polymerized in the presence of the core polymers. First the epoxy resin is dissolved in the core monomer, i.e. vinyl acetate, which is then emulsified and polymerized or copolymerized, as in the case of copolymers of vinyl acetate and ethylene. This forms the "core". The "shell" is then formed by polymerizing an ethylenically unsaturated monomer plus 0.5 to 10 weight percent of the hydroxyl or carboxyl functional monomer in the presence of the core. An amine hardener, such as isophoronediamine, can then be added without premature coagulation. In a comparative example of this patent, this hardener was added to the core polymer without the shell and the mixture gelled within five hours. When the hardener was added after formation of the shell the latex remained stable for more than one month. It is taught that the diamine cannot even be added during the formation of the shell for to do so results in coagulation of the latex.
U.S Pat. No. 5,389,703, Lee (1995) describes still another method of creating a one-pot system of epoxy resin and polymer of ethylenically unsaturated monomer such as an acrylate. These hybrid polymers are made by preparing (1) an organic phase of the epoxy resin and the monomers using an oil soluble surfactant, and (2) an aqueous phase containing a water soluble surfactant. These phases are then combined and the monomers are polymerized to form a water-based emulsion. Even after addition of a hardener such as ethylenediamine the dispersion remained stable without gelation for 4 to 6 months. Omitting either the oil soluble surfactant or the water-soluble surfactant resulted in the formation of coagulum during the polymerization.
A good discussion of the possibilities offered by the combination of epoxy resins with latex emulsions is given in an article by G. C. Young, "Modifying Latex Emulsions With Epoxy Resin Dispersions", Adhesives Age, Sep. 1996, pp. 24-27. Evaluations are described for several types of polymer modifications by the addition of water-based epoxy resin dispersions, for example in the range of 10 to 20 weight percent epoxy resin, to latex emulsions of polymers of vinyl acetate/ethylene, polyvinyl acetate, ethylene/vinyl chloride, polyvinyl chloride, and acrylic polymer. The control of pH of the water-borne epoxy dispersion is said to be important in preventing agglomeration of the latex. The epoxy can be cured in the latex system by addition of amine curing agents. Dicyanodiamide is said to be a latent curative and can produce one-package systems. On the other hand, aliphatic, cycloaliphatic and amido amines are said to react quickly at ambient conditions to produce fully cured networks in one to seven days. Although the potential of water-borne epoxy/latex polymer dispersions is highly touted by this article, it does not provide much help toward solving the problem of long term stability in one-pot systems which are needed by the industry.